Model of the most abundant DNA interstrand cross-link of Transplatin: X-ray structures of two modifications and H bonding behavior in the solid state and in solution of trans-[Pt(NH3)2(1-MeC-N3)(9-EtGH-N7)](ClO4) 2ĚnH2O (1-MeC=1-methylcytosine; 9-EtGH=9-ethylguanine)